The Crusades were a series of holy wars fueled by two major causes: the Pope's desire to expand the Holy Roman Empire and the reclamation of the Holy Land. In the 11th century, Christians and Muslims were at odds over the ownership of Jerusalem, a holy site for both Christianity and Islam.
According to PBS, the Crusades officially began in the year 1095 after Pope Urban II called for Christians to reclaim Jerusalem. There were eight Crusades in total, and although the reasons for the initiation of each Crusade varied slightly, the overarching themes remained essentially the same. The Pope sought to unite Christians across the continent, and he chose the popular idea of reclaiming the Holy Land as his rallying cause.
Although not as frequently cited, the expansion of the Holy Roman Empire was a far more pressing motivation for the Crusades among the political and religious leaders of the day. The wars continued over a period of 200 years throughout the Middle Ages as the empire continued to expand. As explained in the History Channel's online library, the Crusades were also used as a tool to increase papal authority amid the looming threat of schism within the Catholic Church.